Many olefin polymerization catalysts are known, including single-site and Ziegler-Natta catalysts. Both types are based upon transition metals. Typically, other metallic compounds are also used. For example, single-site catalysts generally use aluminum compounds as activators and Ziegler-Natta catalysts typically use an aluminum compound that serves as a cocatalyst. Other metals, such as compounds of magnesium or zinc, are also commonly used in ethylene polymerizations. Generally, these metal contaminants are left in the polyethylene. Depending upon the amount, this can present a problem for certain end-use applications.
Residual metal contaminants are more of an issue with polyethylene prepared by certain processes. For example, solution processes are performed at relatively high temperatures and often require higher catalyst loadings. There have been several attempts to solve this problem by adding various compounds and passing them through an adsorbent such as alumina. Other attempts add a catalyst deactivator and either use other separation techniques or do not remove the metal contaminants.
U.S. Pat. No. 3,281,399, for example, adds an alkylene oxide to a solution of polyethylene and passes the solution over a bed of activated clay. In one embodiment, the alkylene oxide is used in conjunction with a dicarbonyl compound. U.S. Pat. No. 2,978,442 uses acetylacetone to chelate the metal and passes the solution of polyethylene through an adsorbent column. U.S. Pat. No. 4,156,075 discloses a procedure involving boiling or washing the polyolefin with an alcohol such as butyl alcohol or isobutyl alcohol and then washing metal residues from the polyolefin. U.S. Pat. No. 4,430,488 teaches the addition of a carboxylate salt to a solution of polyethylene prepared by a solution polymerization process. The catalyst residues are not removed by adsorbents. A dilute solution (approximately 0.1% by weight) of acetylacetone in 2-butanol is used in comparative run 1 and the resultant polyethylene is reported to be green. U.S. Pat. No. 4,803,259 teaches a similar process using trialkanolamines as deactivators and no adsorbents. U.S. Pat. No. 5,426,175 adds a mixture of a dicarbonyl compound such as acetylacetone and an alkylene oxide to deactivate the catalyst and solubilize the catalyst residues. The catalyst residues are then washed out of the polymer with a suitable liquid such as n-heptane or a lower alkanol, preferably isopropanol or methanol.
The detrimental effect of metal contaminants has been recognized, and while there have been many attempts to solve this problem, a need remains. There are many compounds that can act as catalyst deactivators, but subsequent removal of the metal contaminants is difficult. Solvent washes are often expensive and inefficient. Some added compounds can have a detrimental effect on the polymer. One example of this is β-diketones such as acetylacetone. They are efficient chelating agents and can solubilize the metal contaminants for subsequent removal. However, they form degradation and condensation products which can cause poor thermal stability and poor color of the treated polyethylene. When alcohols alone are used, they can deactivate the catalyst, but they form insoluble materials that plug the adsorbent.